The telephone lines to a residence in the United States and elsewhere can have common mode voltages of over 100V, and the FCC requires the telephone lines to be isolated from any electric main powered device (such as a PC) connected to the telephone lines (through a modem for example) to prevent damage to the telephone network. 47 CFR 68.302,4 (Oct. 1, 1997 Edition). A data access arrangement (DAA) is specified by the FCC to isolate the telephone lines from electric main powered devices, such as illustrated in FIG. 3. Since the voice band modem signal is limited to the 100 to 3600 Hz band, a DAA can be constructed using a transformer which operates as a bandpass filter to isolate the electric main powered device from the telephone lines.
A smaller size and potentially lower cost solution uses active circuits to communicate with the central telephone offices and various modulation techniques to couple the DAA through small capacitors to the PC.
FIG. 4 shows a known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit of FIG. 4 is disclosed and described fully in U.S. patent application Ser. No. 09/028,061 filed on Feb. 26, 1998, entitled Low Noise Line Powered DAA With Feedback, assigned to the same assignee as the present application, and incorporated herein by reference. The circuit is designed in low voltage CMOS technology and can handle only a small amount of voltage. The main function of the circuit is to take the incoming current ILINE supplied by the telephone company and modulate it with a data signal developed by processing a differential data source signal VD with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q1 as a line modulator, and contains a shunt regulator in series with the line modulator Q1. A sense resistor RS1 is placed in series between the line modulator Q1 and the shunt regulator to monitor the current through the shunt regulator.
The circuit depicted in FIG. 4 works by monitoring the current through sense resistor RS1 with a feedback loop around the amplifier A. Resistors RT1 and RB1 sense the differential voltage across RS1. By setting RT1=RB1, the current through RT1 and RB1 will accurately model the current through RS1. The desired signal to be modulated is introduced by a differential data source signal VD. The differential signal is created by adding signal VD/2 to VCM to create VP and subtracting VD/2 from VCM to create VN. This differential signal then drives the input resistors RIP and RIN to provide a differential signal input current. The generation of the differential signal current is well known in the art and will not be further discussed herein. The control amplifier operates to force the current through resistor RS1 to equal the desired signal current by regulating transistor Q2 to control the base of transistor Q1, which in turn regulates the current through the source-emitter path of transistor Q1 and thereby through resistor RS1. In this circuit, the collector current of transistor Q1 is well controlled by the control amplifier A. However, this arrangement incurs a degree of error which is problematic for new communication devices such as high speed data modems. The source of the error is due to current that is outside of the path containing the sense resistor RS1. This stray current will be discussed after a brief discussion of FIG. 5.
FIG. 5 depicts an alternative circuit arrangement similar to the circuit depicted in FIG. 4. However, the circuit in FIG. 5 uses the output of amplifier A to control the emitter of transistor Q2, rather than the base of transistor Q2, and thereby the collector current of transistor Q1. As in the circuit depicted in FIG. 5, the collector current of transistor Q1 is well controlled by the control amplifier A. This arrangement also incurs a degree of error which is problematic for new communication devices such as high speed data modems.
The error associated with the previously mentioned circuit designs of FIG. 4 and FIG. 5 will now be discussed. Ideally, the current through RS1 would equal the current, ILINE, introduced to the system by the telephone company. This would allow amplifier A to take all of ILine into account when modulating the differential signal source onto ILine. An error exists in the line modulation devices of FIG. 4 and FIG. 5 due to the inclusion of only part of the total current ILINE through sense resistor RS1. In both circuits, the current from the telephone company is introduced to the system through the emitter of transistor Q1 (hereinafter “IE1”). In the circuits depicted in FIG. 4 and FIG. 5, IE1 is equal to ILINE, the resistances of RT1 and RB1 are two to three hundred thousand ohms, and the resistance of RS1 is 10-20 ohms. Because of the relatively high level of resistance of RT1 and RB1, the current that flows through RT1 and RB1 can be neglected in the circuit analysis. As current flows through the circuits, IE1 is divided into the transistor Q1 base current (hereinafter “IB1”) and the transistor Q1 collector current (hereinafter “IC1”). The collector current IC1 through the resistor RS1 is used by amplifier A in a feedback loop to modulate the desired signal onto ILINE. Since the current IB1 is outside the feedback loop, an error term in the amount of IB1 is introduced to the circuit, that is, IC1 through resistor RS1 is not equal to ILINE, but is equal to IE1−IB1 or ILINE−IB1.
An additional problem arises from IB1 being outside the amplifier feedback path. Since IC1 and IB1 are related by the β of Q1, and the β of a transistor is a function of the actual signal level, the error term introduced by not accounting for current IB1 in the feedback loop is signal dependent. Signal dependent error terms are a source of harmonic distortion which is problematic for communication devices. In order for current 56 k modems (V.90 standard) to function, a signal to distortion ratio greater than 80 dB is needed. Unfortunately, due to the error term introduced by neglecting IB1, the circuits of FIG. 4 and FIG. 5 can provide a signal to distortion ratio of only about 75 dB, even when high quality components are utilized.
One method which has been used to reduce distortion is depicted in FIG. 6. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/280,473 filed on Mar. 30, 1999, entitled Method and Apparatus for Decreasing Distortion in a Line Powered Modulator Circuit, assigned to the same assignee as the present application, and incorporated herein by reference.
The circuit in FIG. 6 reduces distortion by incorporating a larger portion of ILINE into the feedback path of the control amplifier A. A larger portion of ILINE is incorporated by including a second sense resistor RS2 in a second feedback path to amplifier A in order to sense current introduced to the system by ILINE which does not flow through the first sense resistor RS1. The operation of the differential signal source and the shunt regulator are similar to the differential signal source and shunt regulator discussed above. In addition, as with RT1 and RB1, RT2 and RB2 have a relatively high level of resistance and the current that flows through RT2 and RB2 can be neglected in the circuit analysis.
In FIG. 6, the output of amplifier A is electrically connected to the emitter of transistor Q2 through the additional sense resistor RS2, the collector of transistor Q2 is electrically connected to the base of transistor Q1, and the base of transistor Q2 is electrically connected to the collector of transistor Q1. In this configuration, the original sense resistor current IS1 through the primary sense resistor RS1 is equal to the transistor Q1 collector current IC1 less the transistor Q2 base current IB2. Accordingly, the transistor Q2 base current IB2 equals the transistor Q1 collector current IC1 less the original sense resistor current IS1. The current through the additional sense resistor RS2 is the transistor Q2 emitter current IE2, or equivalently the sum of the transistor Q2 base current IB2 and collector current IC2. Since the transistor Q2 collector current IC2 equals the transistor Q1 base current IB1, the current through the additional sense resistor Q2 can also be said to be the sum of the currents IB1 and IB2. Therefore, the sum of the currents through both sense resistors RS1 and RS2 equals (IB1+IB2+IC1−IB2), or equivalently IB1+IC1, which equals ILINE. This arrangement results in a circuit which is virtually free from distortion. The circuit is free from distortion because the first sense resistor RS1 senses the current and its associated distortion through the shunt regulator, and the second sense resistor RS2 senses all other significant currents and their associated distortion, allowing the amplifier to control ILINE by incorporating all of ILINE in a feedback path.
This method and apparatus for reducing distortion in a line powered DAA requires the amplifier to sense the level of current at multiple locations. Since the level of current through a resistor depends on the resistance of the resistor, the resistors at the various locations must be carefully matched in order to obtain an accurate relationship between ILINE and the current sensed by the amplifier. In addition, the introduction of each additional sense resistor requires the addition of high value resistors such as RT2 and RB2.